1. Field of the Invention
The present invention relates to transmission assemblies for bicycles, and especially to multi-geared transmission assemblies which provide selectable velocity ratios.
2. Description of the Related Art
Bicycles are lightweight, two-wheeled, steerable machines which have been used for transportation, racing, and touring throughout the world over the past two centuries.
Bicycles are propelled by a rider who applies a force to the bicycle transmission assembly using a crank. The transmission assembly is then used to transmit the force to the rear wheel where it is used to drive the bicycle.
Therefore, efficient transmission assemblies which are capable of improving the bicycle speed for a given pedalling rate are essential. For this reason, two types of transmission assemblies have been developed: the epicyclic, which alters the speed of the driver sprocket relative to the rim of the wheel, and the derailleur, which requires movement of the chain from sprocket to sprocket.
Epicyclic gears are made in two-, three-, four-, and five-speed models incorporated in the rear hub. They are controlled by levers or twist-grip mechanisms, utilizing cables mounted on the handlebars or on the frame tubes. Two-speed hub gears are most often made in brake-hub form, controlled by a rotating sprocket in the reverse direction.
The conventional derailleur gears consist of a mechanism to move the chain from one sprocket wheel to another of different size. By varying the size of the driving sprockets, the rear wheel can be made to pass through more or fewer revolutions for each turn of the crank. Cyclists can use up to six sprocket wheels on the rear freewheel and three on the crank. The mechanism is spring loaded to absorb chain slack and is controlled by a cable from the frame or handlebars.
As shown in FIG. 8, the above-described conventional bicycle includes the following parts: a transmission assembly, front and rear wheels 86 and 87, pedal 72, seat 81, handle 84, and frame 80 which includes rear wheel tip 74, seat stay 82, seat tube 71, fork 85, fork tip 75, down tube 88, and an optional crossbar 83.
A bicycle transmission assembly which incorporates the derailleur mechanism includes front and rear sprocket assemblies 50 and 51 rotatably coupled to frame 80 for receiving chain 73. The transmission assembly also includes front and rear derailleur assemblies 53 and 54 for enabling chain 73 to be shifted from one sprocket to another. Chain 73 is then included in the transmission assembly for transferring force from the front sprocket assembly 50 to the rear sprocket assembly 51.
The front sprocket assembly frequently incorporates a two-stage sprocket to provide both high gear 60 and low gear 61 as shown in FIG. 6. Assembly 50 is mounted between the lower portion of seat tube 71 and the base of pedal 72, being rotated about fixed shaft 56.
A front derailleur is also provided for shifting between the sprockets. Derailleur 53 is connected to seat tube 71 by connecting band 62. It includes chain guide 65 which supports chain 73 at the level of the highest sprocket 60. Link means is used to pivot derailleur 53 about hinge 59', thereby moving chain guide 65 laterally over high and low speed sprockets 61 and 60.
When chain guide 65 is shifted, chain 73 is forced toward the adjacent or receiving sprocket until a cog of the adjacent sprocket catches it. Chain 73 is then completely transferred to the receiving sprocket after one full rotation at which time the shift is completed.
The rear transmission means is similar in style to the front transmission means described above since both incorporate multi-stage sprocket assemblies. However, multi-stage rear sprocket assembly 51 often incorporates up to six stages in its multi-stage sprocket as shown in FIG. 7.
The rear transmission means includes multi-stage rear sprocket 52 which is mounted between hub 55 and rear tip 74, where it rotates about fixed shaft 56. It also includes rear derailleur assembly 54 which is mounted below and outside rear tip 74. Finally, chain 73 connects guide gear 57 to multi-stage rear sprocket 52.
A cable (not shown) is then used to apply or release pressure on rear derailleur 54 causing it to pivot around hinge 59 thereby realigning chain 73 with a different sprocket. Chain 73 is then shifted and transferred to the receiving sprocket after one full rotation has been completed.
Multi-stage sprockets of varying diameters provide a means of improving the bicycle speed for a given pedalling rate. However, using them requires the chain to be a distance from the hub greater than the largest sprocket perimeter. Consequently, there is a large distance between the chain and the smaller sprockets. This results in decreased control and frequent chain derailling.
Multi-stage sprockets of varying diameters are also deficient in terms of manufacturing due to their complexity.